The group is led by Bob Fu, an American-based human rights campaigner and friend of Mr Chen.

On Friday, Hu Jia - another friend of Mr Chen and himself a prominent activist and dissident - told the BBC he had met Mr Chen in the US embassy in Beijiing in the last 72 hours, after his escape from house arrest in the eastern province of Shandong. He said Mr Chen had scaled a high wall before being driven hundreds of kilometres to Beijing.

On Saturday, Mr Hu's wife Zeng Jinyan said on twitter that he had been taken away for questioning by local police.

Demands

Mr Chen escaped on Sunday, activists say, and has since released a video addressed to Premier Wen Jiabao.

There are reports that his brother and nephew, and others who helped him escape, have now been detained by police.

The rights group Human Rights in China quoted a source who knew about Mr Chen, and said his nephew Chen Kegui was taken away from his home by more than 30 police officers.

UN human rights chief Navi Pillay said she was concerned for the well-being of Mr Chen and his family, who live in Dongshigu town, Shandong province.

"I'm disturbed to hear reports that other family members, including his brother Chen Guangfu and nephew Chen Kegui, have now been detained," she said in a statement.

Chen Guangcheng

Born 12 Nov 1971

Nickname: The Barefoot Lawyer

Went blind as a child

Campaigned for women forced to have abortions or sterilisation under China's one child per family policy

Jailed for four years in 2006 for disrupting traffic and damaging property

Chen Guangcheng, 40, was placed under house arrest after being released from a four-year jail sentence in 2010. Reports suggest authorities only realised he had escaped on Thursday.

In his video addressed to Premier Wen, delivered from a darkened room, Mr Chen said outwitting his guards had not been easy.

In the appeal, posted online by Boxun, a Chinese dissident news website based in the United States, he asks that:

Premier Wen investigate and prosecute local officials Mr Chen says beat up his family members

The safety of his family be ensured

Corruption in general in China be dealt with and punished according to the law

The Chinese authorities have come under international criticism for their treatment of him. At one point his daughter was barred from school. Many sympathisers who have tried to visit his home say they have been beaten up.

A self-schooled legal activist, Mr Chen is known for revealing rights abuses under China's one-child policy and has accused officials in Shandong province of forcing 7,000 women into abortions or sterilisations.

He Peirong, another China-based activist who had also campaigned for Chen Guangcheng, has also been detained at her home in Nanjing, according to other activists.

The Chen affair comes at an unwelcome time for China's leaders, who have been embroiled in a lurid political scandal involving disgraced former party boss Bo Xilai.